Since 2003 Robert A. Hayes and B. J. Feenstra, from Philips Electronics N.V., published the earliest article about the electrowetting display on NATURE journal, the research about the electrowetting display has been developed increasingly because the structure and the material of the electrowetting display is simple and the manufacturing of the electrowetting display is uncomplicated.
Generally speaking, the electrowetting display includes polar liquid, hydrophobic ink, hydrophobic insulating layers, hydrophilic sidewalls, and transparent electrodes. The contact angle between the hydrophobic ink and the hydrophobic insulating layers may be changed by applying different voltages so as to control the color of the electrowetting display to be transformed.
When the voltage is not applied to the electrowetting display, the polar liquid is repelled by the surface of the hydrophobic insulating layers such that the hydrophobic ink is covered by the polar liquid so as to be evenly distributed. Otherwise, when the voltage is applied to the electrowetting unit, the charge induced by the voltage may attract the polar liquid so that the hydrophobic ink is pushed away by the polar liquid. Therefore, a bright mode and a dark mode of the electrowetting display are achieved by the polar liquid covering the hydrophobic ink or the polar liquid pushing the hydrophobic ink away. Furthermore, the moving region of the hydrophobic ink is restricted by the hydrophilic sidewalls so that different pixels are isolated.
According to the structure of the electrowetting display described above, the hydrophobic insulating layers play an important role in the electrowetting display. The hydrophobicity of the hydrophobic insulating layers must be good enough so that the polar liquid may be repelled by the surface of the hydrophobic insulating layers and the hydrophobic ink may be covered by the polar liquid. On the other hand, the hydrophobic insulating layers must be connected to the hydrophilic sidewalls. However, the surface energy of the hydrophobic insulating layers cannot match the surface energy of the hydrophilic sidewalls, which leads to bad attachment of the hydrophilic sidewalls to the hydrophobic insulating layers as well as hard manufacturing of the electrowetting display for large area. Moreover, the material with superior hydrophilicity cannot be used for the hydrophilic sidewalls, which limits the hydrophilicity of the sidewalls and may lead to oil overflow while driving. Also, designers can not reduce the thickness of the electrowetting display because higher sidewalls are required to prevent oil from overflowing.
Therefore, now designers need to solve the problem met in the previous electrowetting display that the surface tension mismatch between the hydrophobic insulating layers and the hydrophilic sidewalls and the thickness of the electrowetting display cannot be further reduced.